1. Field of the Invention
The present invention relates to exposure control devices used in copying machines, and more particularly to an exposure control device which can erase a foggy zone at the top or end portion of an image.
2. Description of the Prior Art
In an electrophotographic copying machine, an original document image is scanned optically and projected onto a photosensitive member to form an electrostatic latent image thereon, and the electrostatic latent image is developed by toners and the toner image is transferred on a copy sheet and then fixed by heat pressing.
In this case, a fixing device which heats and presses a toner image onto a paper by a heating roller is generally used as the fixing means. However, if there exists a toner image at top end portion of a copy sheet during fixing, some trouble will occur such that a conveying portion for the copy sheet is soiled, or toners adhere to a roller at the bottom side of the fixing device and soil the rear face of the copy sheet by adhering thereto, or the copy sheet sticks to the fixing roller and is rolled thereto.
In order to eliminate the trouble, in the prior art, as shown in FIG. 1(a), a white member W of definite width is provided on the top surface of an original document holding glass 3 corresponding to the scan start end of the original document by means of painting or printing, and an original document scale having a width shorter than that of the white member W is overlapped therewith. The top end of the original document OP to be held on the original document holder glass 3 is positioned to overlap with the white member W at portion which is not covered by the original document scale. When the scanning optical system scans the original document, the scan is started from the white member W whereby the top end portion of an electrostatic latent image of the original document formed on the photosensitive member is erased by length "a" so that a toner image is not formed on the top end portion of the copy sheet.
However, since light reflectance of the white member is determined by the material to be used or thickness of a material to constitute the white member, light reflectance in this part is not always coincide with that of the background of the original document. In usual material of the white member W, reflection light quantity when an illumination light is incident on the original document glass from the bottom surface and reflected by the material to appear at the glass bottom surface, is comparable with a reflection light quantity when an original document of slightly dark background having the reflection density of about 0.3 is held on the original document glass. Consequently, when the scanning is performed, the reflection light quantity being incident on the photosensitive member varies at the border between the white member and the top end of the original document, and fog may be produced at the top end portion of the electrostatic latent image to be erased.
Exposure control devices are roughly classified into a system to control the exposure by manual operation and a system where the exposure is controlled automatically by using a light receiving element to measure the light quantity reflected by the original document.
In a copying machine having not only the white member W but also the manual exposure control mechanism, if the exposure is set by the manual operation so that the original document of a usual white base density is suitably copied, the exposure value will become insufficient at the white member W and a foggy zone will be produced at the top end portion of the image to be formed.
On the other hand, also in the automatic exposure control mechanism particularly the real time automatic exposure control mechanism (hereinafter referred to as "real time AE system") a, similar foggy zone is produced. The real time AE system, as disclosed in Japanese Laid Open Patent Application No. 124374/82 for example, continuously changes the exposure value to a suitable value corresponding to the light quantity from a small portion of the original document subjected to the slit exposure,
In a copying machine having the real time AE, when an exposure sensor (hereinafter referred to as "AE sensor") scans the white member W of the original document glass, the exposure value is corrected to a value corresponding to the reflection density 0.3 of this part and the control without fog is performed. When the AE sensor attains to the border between the white member W and the original document surface OP and the light quantity in this part is detected, if there is diffrence of the reflection light quantity between the white member W and the original document OP, the AE sensor outputs the light quantity detecting signal as shown in FIG. 1(b). In order to make the reflection light quantity constant, the AE system varies the light quantity of the exposure lamp as shown in FIG. 1(c). As a result, the reflection light quantity being incident on the photosensitive member surface extremely varies at the border between the white member W and the original document surface OP as shown in FIG. 1(d). In the original document of the white ground, since the light quantity is once decreased and then increases rapidly, as explained above the fog is produced.